1. Field of Invention
This invention pertains to method and apparatus for recording and reading information stored in helical stripes on magnetic tape, and particularly to method and apparatus tracking servo signals embedded in helical stripes on magnetic tape.
2. Prior Art and Other Considerations
Numerous prior art patents teach the recording and reading of information stored in helical stripes on magnetic tape. In a helical scan arrangement, travelling magnetic tape is partially wrapped around a rotating drum so that heads positioned on the drum are contiguous to the drum as the drum is rotated. A write head on the drum physically records data on the tape in a series of discrete stripes oriented at an angle with respect to the direction of tape travel. The data is formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout.
Among such prior art teachings are the following United States Patents, all commonly assigned herewith and incorporated herein by reference:
(1) U.S. Pat. No. 4,843,495 to Georgis et al., entitled CYCLICAL SERVO ZONE TRACKING METHOD AND APPARATUS FOR HELICAL SCAN RECORDING-DEVICES; PA0 (2) U.S. Pat. No. 4,835,628 to Hinz et al., entitled APPARATUS AND METHOD FOR FORMATTING AND RECORDING DIGITAL DATA ON MAGNETIC TAPE; and, PA0 (3) U.S. Pat. No. 4,845,577 to Georgis et al., entitled APPARATUS AND METHOD FOR ENABLING RAPID SEARCHING OF HELICALLY RECORDED MAGNETIC TAPE.
Helical scan recorders require that the read and write heads be correctly aligned with the stripes of recorded information on the magnetic tape. To this end, servo signals, sometimes referred to as "pilot" signals, are embedded in servo "zones" in each stripe. Typically there are a plurality of servo zones on each stripe.
In some prior art helical scan systems, such as U.S. Pat. No. 4,843,495 to Georgis et al. referenced above, a read head dually functions to detect servo signals provided in a stripe and to read the informational data provided in the stripe. Upon detection of the servo signals by the read head, the system determines if the position of the heads on the drum need be adjusted for correct alignment with the stripes. In other prior art helical scan systems, a special servo head is provided on the drum to detect the servo signals and determine whether head adjustment is needed.
Dual azimuth helical scan arrangements are also known. In dual azimuth helical scan systems, a first head reads or writes stripes at a first azimuthal angle, while a second head reads or writes stripes at a second azimuthal angle. A plurality of servo zones is provided in each stripe. Examples of such dual azimuth helical scan servo arrangements include U.S. Pat. No. 4,528,605 to Hiraguri, U.S. Pat. No. 4,581,662 to Sato, and U.S. Pat. No. 4,665,447 to Odaka.
In the prior art, the servo signals are usually of very low frequency, e.g., a frequency far below the frequency range used for recording data. The prior art shows servo signals written at a low frequency, so as not to be confused with data and not to be subject to azimuth loss, especially in embodiments wherein a read head also serves as a servo head. Unfortunately, low frequency signals are difficult to erase, particularly by overwriting. This limits the number of times a tape can be written, and reduces servo integrity each time.
Prior art helical scan servo systems also consume a significant extent of the tape format. In this respect, the low frequency servo signals must be written in large servo zones, since the number of signal transitions per stripe length is low. In addition, in the prior art the servo zones are conventionally written on each stripe.
In the prior art, servo heads typically attempt to evenly straddle two adjacent stripes. The servo head compares the amplitudes of the signals derived from the servo zones of the two adjacent stripes. In the prior art, if the servo head is off-track by one half track pitch or less, the positional adjustment is relatively easily made. In this regard, in a region wherein the servo head is off-track by one half the track pitch or less, there is a linear relationship between the difference of the compared amplitudes and the extend of required displacement of the servo head to equalize those amplitudes. Thus, in this linear region, a positional adjustment can easily be made to equalize the amplitudes of the servo signals from the two stripes.
Difficulties arise when the servo head is off-track by more than one half the track pitch. The non-linearity hampers a determination of the extent and direction of necessary servo movement in order to obtain the desired on-track condition. The mere comparison between the two servo signal amplitudes does not yield a result that can be used as input for the servo repositioning mechanism.
Accordingly, it is an object of the present invention to provide an accurate and efficient servo method and apparatus for a helical scan recorder.
An advantage of the present invention is the provision of servo tracking method and apparatus that accurately and efficiently determines the degree of correction required in order to properly align heads with a helical stripe on magnetic tape.
Another advantage of the present invention is the provision of servo tracking method and apparatus which reduces the degree of tape format dedicated to servo tracking.
Yet another advantage of the present invention is the provision of servo tracking method and apparatus which facilitates the use of a broad spectrum of frequencies for use as servo signals.